This invention relates to a control system for controlling the outlet or throttle pressure of a boiler so as to maintain it at a desired value while also maintaining the steam flow to the load, such as turbine input, and the boiler input in balance at the same time. This invention is particularly applicable to the drum type boiler and may be useful in industrial boilers as well as in boilers operated in combination with a turbine in such a way that the throttle valve, which adjusts the flow of steam from the boiler to the turbine, is controlled to adjust the turbine input so as to produce the desired output from the generator driven by the turbine. The control system may be of the type which operates to maintain the throttle pressure constant at a predetermined value under varying load conditions on the turbine-generator or it may be of the type which operates to maintain a constant opening on the throttle valve as the load changes while allowing the throttle pressure to vary with the load changes.
When the fuel supply system for the boiler is of the type which makes it difficult to make a direct measurement of the rate at which heat is supplied to the boiler, it is necessary to make an inferential measurement of the heat input to the boiler. One example of such a system is a coal firing system in which the coal is supplied by a coal mill, particularly when the coal may be changed from one grade to another with the different grades having different BTU content.
The problems which arise when applying prior art control systems to coal fired boilers are discussed in U.S. patent application Ser. No. 892,537 filed by T. W. Jenkins and J. C. Barber, now U.S. Pat. No. 4,174,618. Those inventors have, in that application, described one solution to the problem which arises from the frequently used cascade control system wherein the control loop which involves the secondary controller is not substantially faster than the control loop involving the primary controller so that there tends to be an interaction between the control loops.
The system shown and described in the above mentioned patent application is illustrated in connection with a boiler-turbine system wherein the throttle pressure is maintained constant. Other systems in the prior art such as that described in U.S. Pat. No. 3,802,189 issued to Theron W. Jenkins, Jr. on Apr. 9, 1974, describe a control system for controlling the boiler inputs in a boiler turbine combination operating in a sliding pressure mode with the throttle valve being controlled to maintain the desired generator output. The system of that patent, however, is illustrated with reference only to fuels of the type which are easily metered such as oil or gas.
It is an object of this invention to provide an improved boiler control system which will avoid completely the problems which may arise in the prior art system which utilize cascade control.